Heat treating nickel-iron alloys



Jan. 19, 1950 R, G, ASPDEN ETAL 2,921,878

HEAT TREATING NICKEL-IRON ALLOYS Filed Feb. 12, 1958 INVENTORS '4free/Mey dce MAT TREATING NICKEL-IRON ALLOYS Robert G. Aspden and Narayaua I. Ananthanarayanau,

Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Penn- Sylvania Application February 12, 1958, Serial No. 714,926

Claims. (Cl. 148-120) This invention relates to the production of magnetic materials and in particular it concerns a heat treating procedure to consistently develop outstanding properties in magnetic materials.

The use of magnetic nickel-iron alloys for electrical applications is well established commercially. One dimculty in this art, however, has been the inability to obtain consistently satisfactory magnetic properties in the resultant products. Materials seemingly identical and processed in the same manner but taken from dierent batches have not been found to have similar magnetic characteristics after being subjected to conventional heat treatment. The present invention is a method whereby this ditiiculty may be overcome.

lt is a major object of the present invention to provide a method by which nickel-iron alloys having certain oxidizable additions may be produced with consistent magnetic characteristics by a two stage heat treatment, rst by a mild oxidation anneal, which is followed by a reducing anneal.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

in accordance with our discoveries, nickel-iron alloys containing certain oxidizable additions are processed with a double cycle annealing procedure. More specically, the nickel-iron alloy is severely cold rolled to produce sheet which is thereafter annealed under mild oxidizing conditions and then further annealed under reducing conditions. In this manner we have been able to produce nickel-iron materials for use in electrical applications which are characterized by: (l) a square or substantially square hysteresis loop and (2) a controlled loop width as required by the application. Of particular importance, these advantageous results are entirely reproducible. The materials thus produced, as is apparent to those in the art, can be used in the same manner that similar magnetic nickel-iron materials have been used heretofore, for example, as the core materials of self-saturating magnetic amplifiers, or in current transductors.

The cold rolling step of the present invention is conventional with the nickel-iron materials that are now produced. Severe cold rolling as used in this invention indicates a reduction in thickness of the material being worked on of at least 90 percent and frequently as much as 98 percent. As many passes as desired may be used to effect this result as determined primarily by the convenience of the operator and the available equipment,

After the final thickness of 1/2 to 14 mils, or preferably l to 5 mils, has been obtained by cold rolling, the material is oxidized under mild conditions. The term mild oxidation as used herein is intended to indicate oxidation annealing under conditions whereby the nickel and iron are not appreciably affected but where the additions present in small amounts are oxidized. This step is carried out at an elevated temperature but below that at which secondary recrystallization occurs. The mildly oxidizing conditions at the surface of the alloy may be obtained in atmosphere such as: (l) hydrogen, (2) inert gases, (3)

mixtures of hydrogen and inert gases and (4) in a partial vacuum. The oxygen partial pressure of these atmospheres at the annealing temperature must be sucient to oxidize the minor additions, i.e. must exceed the decomposition oxygen partial pressure of those oxides at the annealing temperature. lt is our belief that the function of this step is to diuse oxygen throughout the material and thereby produce a fine oxide dispersion. A typical procedure effective for this step is to heat the nickel-iron sheet for about one half to three or more hours in wet commercial grade hydrogen (dew point say of about +5 -to +35 C.) at a temperature of about 800 to 1300 C.

The second step of the double anneal cycle is a neutral or reducing anneal at an elevated temperature but below the secondary recrystallization temperature. The atmosphere at the surface of the sheet during this anneal may be any of the above atmospheres but with oxygen partial pressures equal to or less than the decomposition oxygen partial pressure of the oxides of the minor elements at the annealing temperature. Typically this involves subjecting the alloy sheet, previously oxidized, to dry commercially pure hydrogen (dew point at least as low as 35 C.) at about 1000 to 1400 C. for at least about one-quarter hour and suitably for about two to six hours. The material is then used in the same manner as is conventional for magnetic nickel-irons now known.

rihis double cycle anneal may be conducted as two separate anneals or as one continuous anneal. Two separate anneals involve annealing in a mildly oxidizing atmosphere, cooling to near room temperature and then annealing in a neutral or reducing atmosphere. The continuous anneal eliminates cooling of the alloy to near room temperature between the iirst and second stage. At the beginning of the second stage, the atmosphere is changed and the temperature may be increased if a higher final annealing temperature is desired.

The alloys useful in the present invention consist essentially of, by weight, 40 to 80 percent nickel, and a total of from more than0.1 percent to about 2 percent of at least one of manganese, aluminum and silicon, and the remainder iron plus the usual impurities in cornmon amounts. Alloying constituents such as manganese, aluminum and silicon are present normally from use as deoxidizers in making the alloys, or as an additive to improve the rolling characteristics. A particularly satisfactory alloy contains 45 to 55 percent of nickel, 0.3 to 2 percent total of manganese, aluminum and slicon and the remainder iron and incidental impurities. The alloys are in the form of sheet of about 0.12 to 0.18 inch in thickness. These alloys are well known and may be made for use in this invention in the conventional manner, for example, by air melting the constituents, deoxidizing the melt and then pouring an ingot. The ingot is then hot rolled to the desired thickness.

The invention will be described further by means of the following specic examples. It should be understood that the details disclosed are by way of illustration and are not to be construed as limiting the invention.

In these examples 0.150 inch thick alloy sheet was used. lt has the following nominal composition in weight percent:

Mo 0.001 Cr 0.01

Patented Jan. 19, 1960 cu 0.03 A1 0.02 Fe Bal.

The plate was cold rolled at near room temperature to a thickness of 0.002 inch, in air to produce a tape.

Example I A sample of the tape was then placed in a furnace at a temperature of l200 C., through which was circulated wet hydrogen (-l-l9 C. dew point), and annealed for two hours. Then the atmosphere was changed to dry hydrogen (-5 0 C. dew point) and the anneal continued for an additional four hours.

Example II Another sample of the cold rolled tape was placed in a furnace at a temperature of 900 C., through which was circulated wet hydrogen (+19 C. dew point), and annealed for 1/z hour. Subsequently, the resulting tape was cooled to near room temperature and then annealed at 1200 C. for 1 hour in a dry hydrogen (-50 C. dew point) atmosphere.

The resulting materials from each of the above examples were tested at room temperature, as insulated tape-wound toroidal cores, with routine A.C. magnetic tests. The materials were found to have a substantially square hysteresisV loop.

The efficacy Vof the invention was also shown by the following series of tests. Alloy materials from two separate batches of nickel-iron, which were slightly, but not materially diierent from the above composition, which were made according to the same melting and processing conditions, were used. The material of the rst batch, hereinafter designated alloy A, was known to be useful after conventional annealing treatment as a magnetic material characterized by a square hysteresis loop. The alloy from the second batch, hereinafter designated alloy B, was known to be incapable of evidencing a square hysteresis loop after being vsubjected to the conventional annealing schedule. Samples of alloy A and alloy B were cold reduced 9S percent to 2 mil tapes and then annealed at 1200 C. in dry hydrogen for four hours. Other samples of alloy A and of alloy B were cold reduced 98 percent to 2 mil tapes and then subjected to the double cycle fanneals described in the examples above. Hysteresis loops on each of these materials were obtained and plotted. The first and second quadrants of these loops are shown diagrammatically in the attached drawing. From the drawing will be noted the characteristic square hysteresis loop of the satisfactory nickel-irons heretofore known. The hysteresis loop of alloy B prepared by the conventional technique gives a loop far removed from ,theV desirable square loop. As shown, however, the double cycle anneals successfully characterized the resulting materials with loops much improved over that of the loop of alloy B resulting upon a conventional anneal las evidenced by an increased squareness of the hysteresis loops.

These unique results are believed to be produced by suppression of the abnormal grain growth. VMore specifically, it is believed that during the mild oxidizing anneal, the trace and alloying elements are oxidized while the nickel and iron are not appreciably affected. Small v oxide inclusions of the trace elements are formed in a nely dispersed condition. Upon subsequent annealing in dry hydrogen, some of these'oxides impede grain boundary `mobility and thereby suppress the abnormal grain growth that could otherwise have occurred.

It willrbe noted that narrower and squarer loops Vresult from the process of Example II.V The lower temperature during the oxidation stage is believed to be the primary cause for the improved magnetic properties. Therefore, the mild oxidation is preferably carried out'at a tem- 4 perature of from 850'C. to 950 C. for l to 1 hour for optimum results.

The aluminum, silicon and manganese are preferably present in amounts ranging from 0.1% to 0.6% for silicon from 0.1% to 0.8% for manganese and up to 0.6% for aluminum. It is believed that chromium may be present and will form'oxides also that function similarly to the aluminum, manganese and silicon. Therefore up to 0.5% chromium may be substituted for these three elements if desired.

It will be understood that the oxidizable additions should form a fine and uniformly distributed oxide precipitate throughout the sheets during the mild oxidation anneal. Y

In accordance with the provisions of the patent statute, we have explained the principle of our invention and have described what we now believe to represent its best embodiment. However, we desire to have it understood that the invention may be practiced otherwise than as specifically described.

We claim as our invention:

1. The process of preparing nickel-iron sheet for magnetic applications which comprises cold reducing at least percent a plate of an alloy consisting essentially of, by weight, 40 to 80 percent nickel, a total of from 0.1% to 2% by weight of at least one readily oxidizable element selected from the group consisting of manganese, aluminum, chromium and silicon, and the remainder iron and incidental impurities, to produce a sheet of a thickness of up to 14 mils in thickness, annealing the resulting sheet at an elevated temperature below the secondary recrystallization temperature under mild oxidizing conditions whereby the readily oxidizable elements are oxidized but not the iron and nickel, then annealing the sheet in a non-oxidizing atmosphere at an elevated temperature which atmosphere has an oxygen partial pressure not exceeding that of the oxides of the said readily oxidizable elements to develop outstanding magnetic properties therein.

2. The process of preparing nickel-iron sheet for magnetic applications which comprises cold reducing at least 90 percent a plate of an alloy consisting essentially of, by weight, 40 to 80 percent nickel, a total of from 0.1% to 2 percent of at least one relatively easily oxidizable element of the group consisting of manganese, aluminum, chromium and silicon, and the remainder iron and incidental impurities, to produce a sheet of `a thickness of up to 14 mils in thickness, annealing the resulting sheet at an elevated temperature below the secondary recrystallization temperature under mild oxidizing conditions in an atmosphere having an oxygen partial pressure that is higher than the decomposition oxygen partial pressure of the oxides of the readily oxidizable elements present in minor anmounts at the annealing temperature, and then annealing the sheet in 'a non-oxidizing atmosphere at an elevated temperature of at least 1000 C., the oxygen partial pressure of the atmosphere not exceeding that of the said oxides at the annealing temperature, to develop outstanding properties therein.

3. The process according to claim 2 in which said mild oxidizing anneal is carried out in an atmosphere of wet hydrogen having a dew point of from about 5 C. to 35 C.

4. The process of producing nickel-iron sheet for magnetic applications which comprises cold reducing at least 90 percent an alloy plate consisting essentially of, by weight, 40 to 80 percent nickel, a total of up to about 2 percent of at least one relatively readily oxidizable element of the group consisting of manganese, aluminum, chromium and silicon, and the remainder iron and incidental impurities, to produce a sheet of a thickness of up to 14 mils in thickness, annealing the sheet at a temperature of about 800 to l300 C., in wet hydrogen having a dew point of from about 5 C. to 35 C. for about one quarter to 3 hours to oxidize the readily oxidizable elements thereof which are present in small amounts, then annealing the resulting sheet in dry hydrogen of a dew point of less than about -35 C. at a temperature of about l000 to 1400 C. for one quarter `to six hours whereby outstanding magnetic properties are developed therein.

5. The process of producing nickel-iron sheet for magnetic applications which comprises cold reducing at least 90 percent an alloy plate consisting essentially of, by weight, 45% to 55% nickel, from 0.3% to 2% of at least one readily oxidizable element from the group consisting of aluminum, manganese, chromium and silicon, the silicon being in the range of from 0.1% to 0.6%, the manganese being in the range of 0.1% to 0.8%, the chromium being present not in excess of 0.5%, and the aluminum being present in amounts of up to 0.6%, the balance being iron except for incidental impurities, to

produce a sheet of a thickness of up to 14 mils in thickness, annealing the sheets at a temperature of from 850 C. to 950 C. in wet hydrogen having 1a dew point of from about 5 C. to 35 C. for a period of time of from 1/2 hour to l hour to oXidze the oxidizable elements to produce a fine dispersion of oxides throughout the sheets, then annealing the resulting sheet in dry hydrogen of a dew point of less than about -35 C. at a temperature of about 1000 to l400 C. for one quarter to six hours whereby outstanding magnetic properties are developed therein.

Cio July 12, 1932 Bozorth May 28, 1935 

1. THE PROCESS OF PREPARING NICKEL-IRON SHEET FOR MAGNETIC APPLICATIONS WHICH COMPRISES COLD REDUCING AT LEAST 90 PERCENT A PLATE OF AN ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, 40 TO 80 PERCENT NICKEL, A TOTAL OF FROM 0.1% TO 2% BY WEIGHT OF AT LEAST ONE READILY OXIDIZABLE ELEMENT SELECTED FROM THE GROUP CONSISTING OF MANGANESE, ALUMINUM, CHROMIUM AND SILICON, AND THE REMAINDER IRON AND INCIDENTAL IMPURITIES, TO PRODUCE A SHEET OF A THICKNESS OF UP TO 14 MILS IN THICKNESS, ANNEALING THE RESULTING SHEET AT AN ELEVATED TEMPERATURE BELOW THE SECONDARY RECRYSTALLIZATION TEMPERATURE UNDER MILD OXIDIZING CONDI- 